There is known to the art a number of paper-web drying arrangements of the aforedescribed principal construction.
The present invention is based on drying sections or drying arrangements of the kind in which a plurality of blow boxes are located transversally of the longitudinal direction of the paper web and operative to hold the web suspended in a stable position above the boxes. To this end each box has a planar, or substantially planar upper surface having formed therein air-exit orifices, each of which is oriented to direct a stream of air parallel to the web of paper, or some other material.
An example of such constructions is described and illustrated in U.S. patent specification No. 3,231,165.
It is also known to provide an arrangement of top and bottom blow boxes, in order to achieve an improved drying effect, therewith enabling a shorter drying-path through the dryer to be accepted, which in turn enables the coated or lined structural surface of the dryer to be reduced, in the case of new constructions.
An example of a prior-art dryer of this kind is described and illustrated in South African Patent Specification No. 82-7124, this known dryer incorporating a plurality of upper and lower blow boxes which are substantially parallel and at right angles to the direction of web travel. The lower blow boxes have provided in the surfaces thereof facing the web a plurality of air-exit or blow orifices, each of which is arranged to project a stream of air in a direction substantially parallel with the plane of the web, in a manner to support the web.
The speed at which the air-streams exit through the orifices and the configuration of said orifices are such as to enable respective air streams to sustain the web in a given position of suspension above the blow boxes. If the web has a sufficiently high surface weight (in excess of 400 g/m.sup.2), the upper blow boxes can be located on the opposite side of the web, and are normally provided with blow orifices adapted to blow air in a direction substantially at right angles to the plane of the web.
In dryers of the kind described and illustrated in the U.S. patent specification, it has been observed in practice that not all the air streams issuing from the upper blow boxes are directed at right angles to the plane of the web, but that some air streams deviate from this direction and obtain a velocity component which is directed in the longitudinal direction of the blow boxes, parallel with the flow direction of the incoming air-flow, entering said box.
In the case of air-exit orifices formed in the manner described and illustrated in the aforesaid U.S. patent specification, where the orifices are in direct communication with an air-supply chamber or passageway, it has been found that the exiting air stream has a helical configuration, where the major directional axis has a directional component parallel with the velocity vector of the stream of supply-air. The directional component of downstream air-flows, seen in the direction of air flow, becomes smaller and smaller, and consequently mutually adjacent air streams will be directed slightly towards one another.
In dryers of the kind described and illustrated in the aforesaid South African patent specification, it has also been observed with regard to the upper blow boxes, where the air-stream exit orifices are in direct communication with an air-supply chamber or passageway, that the air streams adjacent the end part of the upper blow boxes exit in a direction having a progressively decreasing directional component which is parallel with the velocity vector of the air-supply flows, and hence mutually adjacent air streams are directed slightly towards one another.
These converging air streams, irrespective of whether they are generated by the lower or upper blow boxes, result in irregular increases and decreases in pressure in the region of the surfaces of the blow boxes and create air transportation along said surfaces and transversally of the direction of web travel. It is obvious that web flutter will be greater in the presence of upper and lower airstreams having directional components parallel with the velocity vector of the air-supply stream, than in the presence of solely lower air streams. These two occurrences, pressure variations and air transportation along the boxes, impair stabilization of the web effected through the air streams projected thereagainst, i.e. impair the effect of the forces which strive to hold the web at a constant distance from the blow boxes. The aforesaid occurrences of pressure differences and in transportation also generate forces which act laterally on the web, thereby causing web movement in a lateral direction.
The oblique path followed by the air streams from the upper blow boxes, inter alia due to directional components which lie parallel with the velocity vector of the incoming stream of supply air, are liable to result in a negative, amplifying effect, when local pressure increases on the upper surface of the web occur simultaneously with pressure decreases on the undersurface thereof.
In summary, it can be said that the convergent air streams result in an irregular pressure distribution across the top and bottom surfaces of the web, and that there is obtained a resultant air flow between blow boxes and web which is directed transversally to the direction of web travel.